Removal of vinylacetylenes from alkadienes



United States Patent Oflice 285,01 3 Claims. (Cl. 260681.5)

This invention relates to a process for reducing the concentration of acetylenes in hydrocarbon mixtures, and more particularly, the reduction of the vinylacetylene concentration in C to C alkadienes.

Conjugated alkadienes, particularly butadiene and isoprene, serve as important starting materials in the chemical industry, finding heavy use in polymerization processes, such as the production of various synthetic rubbers. Such diolefins are obtained commercially by cracking higher petroleum fractions, dehydrogenation of the corresponding saturated compounds, or from the refinery light-ends. Cracking and dehydrogenation are, for the most part, the principal source of alkadienes and in such preparations acetylenes are also formed. The presence of acetylenes, particularly vinylacetylene, is undesirable because of the deleterious effect vinylacetylene has on the chemical, especially polymerization, processes involving alkadienes.

Because of the undesirability of vinylacetylenes in alkadienes which have been commercially prepared, a great deal of effort has been expended to find a suitable, economical process for their removal. Two principal approaches to the removal of vinylacetylenes have been selective removal by physical means, such as extraction with solvents which selectively absorb the vinylacetylene, and the alternative, the selective hydrogenation of the vinylacetylenes in the presence of the alkadienes. However, neither of these approaches have been entirely satisfactory because of the excessive alkadiene loss and special equipment requirements.

Accordingly, it is a principal object of the present invention to provide a simple process for satisfactorily solving the problem of obtaining very low vinylacetylene concentrations in alkadienes with present existing eqipment operated at original design throughputs.

Another important objeect of the invention is the provision of a process which results in alkadienes with low vinylacetylene concentrations without undue loss of the alkadienes.

Other objects and advantages of the invention will be apparent from the description thereof which follows.

Generally, this invention consists of a process for reducing the concentration of vinylacetylenes in C to C alkadiene feed streams which involves fractionating the stream to obtain one fraction having a higher concentration of vinylacetylenes and a second fraction having a lower concentration of vinylacetylenes, subjecting such first fraction to hydrogenation conditions in the presence of hydrogen and a hydrogenation catalyst to hydrogenate the vinylacetylene in the presence of and substantially to the exclusion of the alkadienes in such first fraction and subsequently combining said hydrogenated fraction with said second fraction to obtain an alkadiene stream with minimal amounts of vinylacetylenes.

Research demonstrated that the loss of alkadienes in a selective hydrogenation process to eliminate the vinylacetylenes was related to the relative concentration of the acetylenes and the alkadiene in the hydrocarbon stream subjected to hydrogenation. Apparently, the vinylacetylenes are selectively and preferentially absorbed on 3,342,891 Patented Sept. 19, 1967 the catalytically active surfaces and when the concentrations of the vinylacetylenes are relatively high during the hydrogenation process, it was found that very little, if any, alkadiene will be hydrogenated. In fact, it was quite surprising to find that in some circumstances, the resulting concentration of the alkadienes will be increased slightly by this hydrogenation step because of the conversion of the vinylacetylenes to alkadiene and the almost negligible loss of alkadiene from the hydrogenation.

As can be seen, this novel process has largely overcome the difliculties of alkadiene loss involved in selective hydrogenation while providing for the preparation of alkadiene streams having a very low concentration of the undesirable vinylacetylenes. In fact, as mentioned above, it is possible through the proper control of the instant process to purify alkadiene feed streams without loss of alkadiene from the hydrogenation step. Processes described in prior art did not avoid alkadiene loss but tried to minimize the loss by exacting control of the hydrogenation conditions. The new process described herein avoids the alkadiene loss by (1) fractionating the alkadiene feed stream to obtain two fractions, one high in vinylacetylene concentration and one lower in vinylacetylene concentration aud (2) subjecting only that fraction which is high in acetylene concentration to hydrogenation thereby avoiding the necessity of contacting the Whole feed stream in the hydrogenation step, which would increase the alkadiene loss. Since only one portion or fraction of the alkadiene feed stream is subjected to the hydrogenation step, the overall efficiency of the process is substantially improved and the equipment for the hydrogenation step need not be capable of handling the whole through-put of alkadiene feed stream to be purified.

More specifically, the process can be considered as a trilogy of steps. Working with C to C hydrocarbon alkadiene feed streams, the first step of the process is to fractionate such alkadiene feed stream into two portions. The fractionation of the alkadiene stream is accomplished in such a manner that one of the portions of fractions has high vinylacetylene concentration while the other is substantially free of vinylacetylenes. The second step of the process is to subject the fractionated portion having the high vinylacetylene concentration to hydrogenation conditions in the presence of the hydrogenation catalyst and hydrogen to partially hydrogenate the vinylacetylenes. The hydrogenation reduces the concentration of the vinylacetylenes in the fraction having the high concentration of vinylacetylenes to a concentration equal to or below their concentration in the raw alkadiene feed stream. The third step of the process is to combine the feed stream fraction substantially free of acetylenes with the hydrogenated fraction either by directly recombining them or recycling the hydrogenated fraction with the raw alkadiene feed, in order to obtain an olf alkadiene stream with a very low net concentration of vinylacetylenes.

The first step of the process involving the fractionation of the alkadiene feed stream, preferably butadiene or isoprene, to obtain two fractions, one substantially free of vinylacetylenes and a second fraction having a higher concentration of vinylacetylenes can be accomplished in various Ways and is not confined to a particular method. Several of the Various concentration processes which are suitable for the fractionation of alkadiene streams for this step are extractive distillation, chemisorption, and rectifying absorption.

A particularly advantageous embodiment of the vinylacetylene concentration step of the process can be eifected by rectifying absorption with dimethyl sulfoxide as the absorbing agent. This can be conveniently accomplished with a column in which dimethyl sulfoxide is passed counter-currently to a C, and/or C alkadiene feed stream. The dimethyl sulfoxide preferentially absorbs the vinylacetylenes in the stream and can be subsequently stripped or purged with nitrogen to obtain the fraction wtih the high vinylacetylene concentration for subseouent selective hydrogenation.

Another method which can be used in the vinylacetylene concentration step of the process is extractive distillation of the C, and/or C alkadiene feed stream with a polar selective solvent such as furfural, acetone, acetonitrile, and water mixtures thereof.

The alkadiene feed stream contaminated with the vinylacetylenes is fed to a distillation column wherein it is contacted in the polar solvent and from an appropriate column the vinylacetylenes will be recovered as bottoms. Thus, a side stream taken off from the lower portion of the column where the vinylacetylenes are concentrated will be reach in acetylenes and can be subsequently selectively hydrogenated. The tops or overhead will be a substantially vinylacetylene-free portion of the stream. In this manner, an alkadiene stream may be divided into two fractions or portions, one of higher vinylacetylene concentration and the other substantially free of vinylacetylene.

A preferred embodiment for the acetylene concentration step of the process is effected by distillation in which a portion rich in vinylacetylenes is recoverable as bottoms. In this alternative, a side stream having a higher concentration of vinylacetylenes is withdrawn from the lower portion of the distillation column. Alternative designs of distillation columns may be used and a suitable column is one containing approximately 80-160 trays. Of course, it should be appreciated that the number of trays, the size and other particulars of the column used depend largely on the initial vinylacetylene concentration in the alkadiene which is to be purified and, also, on the desired final degree of purity of the alkadiene. Generally an average reflux/feed ratio of 5 to 10, an overhead temperature of 40-60 C. and a pressure of approximately 4.8 to 8 atmospheres absolute are quite satisfactory for this distillation. The point of withdrawal of the side stream depends largely on the column, concentration of vinylacetylenes, and various other conditions, but in general, it is advantageous to choose the withdrawal point in the lower half and probably in the lower third of the distillation column since the concentration of the vinylacetylenes will be higher in this portion of the column.

Subsequent to the fractionation of the alkadienes feed into a fraction substantially free of vinylacetylenes and a fraction having a higher concentration of vinylacetylenes, the second step of the process hydrogenation can be accomplished. In this second step of the process, the fraction of the feed with the higher vinylacetylene concentration is selectively hydrogenated, i.e., hydrogenated in a manner to avoid hydrogenation of alkadienes, in any one of several alternative methods. This hydrogenation requires the presence of a catalyst, such as nickel, cobalt, chromium, magnesium, or vanadium; metal oxides, such as iron oxides, may also be used as hydrogenation catalysts for this step which can be effected in a gaseous or liquid phase. The preferred catalyst is palladium because of its somewhat greater specificity for hydrogenation of the vinylacetylenes. Regardless of which catalyst is chosen,- it is best supported on an inert particulate support, such as alumina or silica. A highly satisfactory catalyst is 5% to .5 palladium by weight on gamma aluminum oxide packed in a hydrogenation column.

Naturally, alkadiene streams should be free of catalyst poisons, such as the thiols and other sulfur compounds since these materials would tend to poison the hydrogenation catalyst and thus affect the net purity of the final feed stream resulting from the recombination of the two fractions.

The hydrogenation can be effected in a gaseous phase, however, the preferred method is a liquid phase hydrogenation. Most satisfactory liquid phase hydrogenation accomplished by passing a solution of the alkadiene stream fraction containing the higher concentration of vinylacetylenes with dissolved hydrogen in upflow through a fixed bed of palladium dispersed on gamma aluminum oxide under sufficient pressure to maintain the liquid phase and at ambient temperature.

Depending on the initial concentration of the vinylacetylenes in the alkadiene feed streams and fractionating method used, the fraction of the stream which is subjected to hydrogenation may contain up to 3.5%, or greater, by volume of vinylacetylenes. It was found that when this fraction was hydrogenated, very little, if any, diolefin was lost because of hydrogenation. In fact, as will be demonstrated by one of the subsequent examples, hydrogenation actually increased the concentration of the alkadiene while substantially reducing the concentrations of vinylacetylenes to a concentration of the original feed stream or lower as a result of the preferential and partial hydrogenation of vinylacetylenes forming some alkadiene.

The third step of the process involves recombining the hydrogenated fraction of the alkadiene feed stream with the unhydrogenated fraction of the stream which results in a stream in which the net concentration of the vinylacetylenes is at an acceptable low level. This can be accomplished by either combining them directly or, alternatively, recycling the efiluent from the hydrogenation step with the raw or contaminated alkadiene feed since the vinylacetylene concentration of the effluent is the same or less than that in the raw alkadiene feed. When the hydrogenated fraction is combined directly with the unhydrogenated fraction of the stream, it was found that the resulting stream contained less than 500 ppm. (parts per million) of vinylacetylenes and further, by careful control of the conditions of the selective hydrogenation conditions, the resulting stream has less than 100 ppm. vinylacetylene concentration.

Where especially high purity alkadiene feed streams are desired, it is advantageous to add the effiuent from the hydrogenation step with the raw alkadiene feed which is entering the process, so that it will result in a recycle of the hydrogenated fraction. Also, in some cases, the net efficiency of the process may be improved by hydrogenation of only a portion of the vinylacetylenes in the fraction of the feed stream containing the higher concentration of vinylacetylenes and recycling the efiluent from the hydrogenation step with the raw alkadiene. Thus, for example, when the vinylacetylenes are concentrated by distillation, the hydrogenation effluent may be advantageously recycled to the distillation column entering with the raw alkadiene feed to obtain the lowest acetylene concentration in the alkadiene 01f stream.

This novel process of especially suitable for hydrocarbon mixtures of C to C cuts of alkadienes having the same number of carbon atoms and can also be used with alkadienes having a greater or lesser number of carbon atoms. It was found that the process was satisfactory for mixtures of C and C hydrocarbons cuts, such as butadiene and isoprene.

The concentration of the alkadiene in the feed stream may vary within wide limits and C and/ or C cuts with alkadiene concentrations of between 20 and 50% by weight are satisfactory, as well as cuts in which the alkadiene concentration may be as high as The process is particularly adaptable to crude alkadiene cuts which contain alkenes since the step involving the concentration of the vinylacetylenes will eliminate a large part of the original volume from the hydrogenation step of the process which gives a net increase in efiiciency and economy. Moreover, the vinylacetylene concentration in the alkadiene stream is not critical and persons skilled in the art can easily calculate the design of the various apparatus needed for the three steps of the process relative to the respective concentrations of the components in the alkadiene streams.

Example I In the acetylene concentration phase of the process, 100 liters of gaseous C alkadiene feed and 720 ml. of dimethyl sulfoxide were passed countercurrently at a temperature of 26 C. through an Odershaw column having 30 trays, a length of 120 centimeters and an internal diameter of approximately 2% centimeters. The C feed contained approximately 3200 p.p.m. of vinylacetylene and a butadiene content of 37.1% by volume. The off-gas fraction of the feed stream contained approximately 21 p.p.m. (parts per million) of vinylacetylene. The dimethyl sulfoxide was stripped with nitrogen to obtain the fraction of the feed having the higher vinylacetylene concentration. On a nitrogen free basis, the stripping gas had a volume of liters and contained 3.5% by volume of vinylacetylenes and 61% by volume of butadiene.

This vinylacetylene concentrate was subsequently hydrogenated in upflow reaction zone packed with a .5% by weight palladium catalyst dispersed on gamma aluminum oxide, at 30 C. and under a pressure of 6 atmospheres absolute. The vinylacetylene concentration in this fraction subjected to hydrogenation was reduced from 3.5% by volume to approximately 3200 p.p.m. (same as that of the original alkadiene feed) as a result of the hydrogenation and the elfiuent was recycled to the adsorption column.

Under these conditions, the loss of butadiene was 5% by volume based on the feed to the hydrogenation step which would correspond to a net loss of 0.5% by volume of butadiene based on the original alkadiene feed.

Example II Data was obtained by the use of a distillation column with internal diameter of approximately 3 meters and provided with 152 trays. The column was operated at a pressure of approximately 6 atmospheres absolute, a top temperature of approximately 45 C., and a bottom temperature at approximately 87 C. An alkadiene feed stream having the composition set forth under column A in Table I was fed to the column at a rate of 40 cubic meters per .hour and at a temperature of 27 C.

Approximately 23 meters above the bottom tray 21 side stream was taken 01f the column as a fraction of the feed having an increased concentration of acetylenes. The side stream was withdrawn at approximately 4 cubic meters per hour and had a temperature of about 75 C. The composition of this side stream appears under B in Table I.

This side stream was passed to a hydrogenation reactor having a volume of approximately 0.4 cubic meter, an internal diameter of 0.3 meter and a length of 5.7 meters. The reactor was of the upflow type and was packed with 0.5% by weight palladium on gamma aluminum oxide. It was operated under a pressure of approximately 6 kilograms per square centimeters absolute. Approximately 0.59 kilogram per hour of hydrogen was mixed with the 4 cubic meters of side stream entering the reaction.

The composition of the efliuent from the hydrogenation reactor is given under C in Table I; it was recycled to the distillation column. It was surprising that the butadiene content was greater subsequent to passage through this hydrogenation step and the effluent from the hydrogenation reactor was evolved at a rate of 4 cubic meters per hour and had a temperature of approximately 40 C.

The overhead product from the distillation column contained 95.5% by weight of butadiene and only 60 parts per million of vinylacetylene which is demonstrative of a process by which the concentration of the vinylacetylenes can be reduced to an acceptable level without appreciable losses of butadiene.

TABLE I Composition, in Percent by Volume Components of the Hydrocarbon Stream We claim as our invention:

1. A process of reducing vinylacetylene concentration to below 100 parts per million in a C or C alkadiene distillation product from a C or C alkadiene feed stream containing 20 to C or C alkadiene, an undesirable proportion of closely boiling vinylacetylenes and the remainder being essentially corresponding C or C alkenes by:

(a) fractionally distilling the alkadiene feed stream in a distillation zone and separating a concentrate stream enriched in and containing at least about 1% and no more than about 3.5% by weight vinylacetylenes;

(b) catalytically hydrogenating the concentrate in upflow through a bed of 0.5% to 5% by weight palladium-on-alumina catalyst to selectively hydrogenate the vinylacetylenes to a concentration of less than parts per million while increasing the concentration of the alkadiene therein;

(c) recycling the hydrogenation elfiuent to the feed stream to the distillation zone; and

(d) withdrawing the alkadiene content of the feed stream as an enriched alkadiene stream of less than 100 parts per million vinylacetylenes from the distillation Zone.

2. A process in accordance with claim 1, wherein the alkadiene feed stream of step (a) is a C alkadiene stream containing about 37.1% by volume butadiene and about 3200 p.p.m. (0.32%) vinylacetylene, and the separated vinylacetylene concentrate of step (a) contains about 3.5 by volume vinylacetylene and about 61% by volume butadiene, and the concentrate is hydrogenated in accordance with step (b) to a vinylacetylene concentration of about 3200 p.p.m. before recycling in accordance with step (c) to the distillation zone.

3. A process in accordance with claim 1, wherein the feed stream of step (a) contains about 22.1% butenes, about 78% butadiene and about 0.045% vinylacetylene; the separated stream of enhanced vinylacetylene concentration of step (a) contains about 50.9% butenes, 48.1% butadiene and 1% vinylacetylene; and the hydrogenated stream of step (b) contains about 48.25% butadiene and 0.6% vinylacetylene.

References Cited UNITED STATES PATENTS 3,000,794 9/ 1961 Tschopp 260-6815 3,070,641 12/1962 Herndon et ;al. 260-6815 3,076,858 2/1963 Frevel et a1. 260-6815 3,091,654 5/1963 Kestner 260-6815 3,200,167 8/1965 Reich 260-6815 DELBERT E. GANTZ, Primary Examiner. G. E. SCHMITKONS, Assistant Examiner. 

1. A PROCESS OF REDUCING VINYLACETYLENE CONCENTRATION TO BELOW 100 PARTS PER MILLION IN A C4 RO C5 ALKADIENE DISTILLATION PRODUCT FROM A C4 OR C5 ALKADIENE FEED STREAM CONTAINING 20 TO 90% C4 OR C5 ALKADIENE, AN UNDESIRABLE PROPORTION OF CLOSELY BOILING VINYLACETYLENES AND THE REMAINDER BEING ESSENTIALLY CORRESPONDING C4 OR C5 ALKENES BY: (A) FRACTIONALLY DISTILLING THE ALKADIENE FEED STREAM IN A DISTILLATION ZONE AND SEPARATING A CONCENTRATE STREAM ENRICHED IN AND CONTAINING AT LEAST ABOUT 1% AND NO MORE THAN ABOUT 3.5% BY WEIGHT VINYLACETYLENES; (B) CATALYSTICALLY HYDROGENATING THE CONCENTRATE IN UPFLOW THROUGH A BED OF 0.5% TO 5% BY WEIGHT PALLADIUM-ON-ALUMINA CATALYST TO SELECTIVELY HYDROGENATE THE VINYLACETYLENES TO A CONCENTRATION OF LESS THAN 100 PARTS PER MILLION WHILE INCREASING THE CONCENTRATION OF THE ALKADIENE THEREIN; (C) RECYCLING THE HYDROGENATION EFFLUENT TO THE FFED STREAM TO THE DISTILLATION ZONE; AND (D) WITHDRAWING THE ALKADIENE CONTENT OF THE FEED STREAM AS AN ENRICHED ALKADIENE STREAM OF LESS THAN 100 PARTS PER MILLION VINYLACETYLENES FROM THE DISTILLATION ZONE. 